An innovative method for studying the dynamic failure mechanism of box connection of stabilizer in large diameter wellbore of ultra-deep wells

Abstract

The SB Oilfield in China is characterized by ultra-deep wells with a depth more than 7000 m. During drilling the large wellbore in upper section, stabilizer failure occurred frequently. Three out of seven ultra-deep wells occurred with a fracture accident at the box connection of stabilizer. These accidents have caused great economic losses to the oil fields. However, the cause of this failure cannot be explained by previous research results. In this paper, we present an innovative method, which combines the analyses of drillstring dynamics and localized stress of tool joint, to study the failure mechanism of the stabilizer box connection and theoretically explain the cause of failure. Firstly, numerical simulation model of drillstring dynamic was carried out, from which we analyzed the dynamic loads on drillstring with focus at the stabilizer. Then an ABAQUS model was established to study the mechanical characteristics of the stabilizer thread connection. Finally, the vibration signal was decomposed by Fourier analysis, the fatigue frequency ratio and vibration frequency spectrums were obtained. On this basis, the fracture mechanism of stabilizer box connection was revealed. The results showed that the unreasonable drilling parameters could cause severe impacts between drillstring and borehole. These impacts were prone to induce high-frequency bending moment at box connection of the stabilizer by noting that large variable cross-sections of drillstring were used in SB Oilfield. The high-frequency stress acting on thread caused the failures of stabilizer. The maximum bending moment at the box connection of stabilizer reaches 196.49 kN m. It results in a maximum difference of von Mises stress being 407.40 MPa at the first engaged thread with a frequency of 27.06 Hz. This leads to a fatigue frequency ratio of 64.58. Such a large fatigue frequency ratio means that the box connection is subject to a large load with a high frequency, which eventually causes the failure of stabilizer. It suggests that the proposed method, based on the analyses of drillstring dynamic stress and localized stress variation of tool joint, is an effective way to study the failure of drillstring stabilizer.